neck reigning is when the horse responds to the touch of the reins on one side of the neck. they move in the same direction the reigns are going, rather than responding to pressure in the mouth.
I watched an episode of neck reining on a TV program of mine. I can't really explain it, but I can give an example.
For instance, if you wanted your horse to turn left, what you would do first is...
When you plow rein you use both hands, when you pull right your horse turns right, and when you pull left your horse turns left, and when neck reining you use one hand (doesn't matter which) and with...